1. Field of the Invention
The present invention relates to a method for producing an aromatic polycarbonate. More particularly, the present invention is concerned with a method for producing an aromatic polycarbonate which comprises subjecting to a transesterification polymerization reaction at least one polymerizable material selected from the group consisting of a molten monomer mixture of an aromatic dihydroxy compound and a diaryl carbonate, and a molten prepolymer obtained by reacting an aromatic dihydroxy compound with a diaryl carbonate, wherein the transesterification polymerization reaction is performed in one or more polymerizers which is or are connected through a pipeline system toward an outlet for a final aromatic polycarbonate product, wherein the pipeline system comprises one or more pipes through which a molten aromatic polycarbonate having a number average molecular weight increased by the transesterification polymerization reaction is passed while contacting an inner wall of the pipe or pipes, and wherein molten aromatic polycarbonates respectively having a number average molecular weight of less than 2,500 and a number average molecular weight of 2,500 or more are passed through the pipe or pipes at velocities of specific values or more, respectively. The method of the present invention is advantageous not only in that a high quality aromatic polycarbonate which is not only highly colorless, but also has a low foreign matter content can be easily produced by using an ordinary pipeline system comprising one or more pipes made of an ordinary material, but also in that there is no need for a step of forcibly passing a molten poly-carbonate (inherently having a high melt viscosity) through a polymer filter by using an extruder, so that problems do not arise, such as clogging of the polymer filter or an increase in load on the extruder.
2. Discussion of Prior Art
In recent years, aromatic polycarbonates have been widely used in various fields as engineering plastics having excellent heat resistance, impact resistance and transparency. Use of aromatic polycarbonates has been rapidly expanding especially as a material for an optical disk substrate. With respect to methods for producing aromatic polycarbonates, various studies have heretofore been made. Of the methods studied, a process utilizing an interfacial polycondensation between an aqueous alkali solution of an aromatic dihydroxy compound, such as 2,2'-bis(4-hydroxyphenyl) propane (hereinafter, frequently referred to as "bisphenol A"), and phosgene in the presence of an organic solvent has been commercially practiced. The organic solvent used for the above-mentioned interfacial polycondensation process is a halogen-containing organic solvent, such as methylene chloride or chlorobenzene. Of these, methylene chloride is usually used.
However, the interfacial polycondensation process has a problem in that difficulties are encountered in separating and removing the organic solvent from the obtained polymer. Therefore, due to the presence of a halogen derived from the remaining organic solvent, problems arise not only in that corrosion occurs in the mold used for the molding of the obtained aromatic polycarbonate, but also in that the obtained aromatic polycarbonate suffers a discoloration, leading to a lowering in quality of the aromatic polycarbonate. Especially when the obtained aromatic polycarbonate is used for an optical disk substrate, the interfacial polycondensation process has a fatal problem in that the remaining halogen in the polycarbonate causes corrosion of the recording layer of such optical disk, thus causing an error in the recorded information.
With respect to a method for producing an aromatic polycarbonate from an aromatic dihydroxy compound and a diaryl carbonate, in a conventionally known melt transesterification process, an aromatic polycarbonate is produced by a transesterification polymerization reaction between bisphenol A and diphenyl carbonate in the molten state, while removing a by-produced phenolic compound (phenol). Unlike the interfacial polycondensation process, the melt transesterification process has an advantage in that a solvent need not be used. However, the melt transesterification process has a serious problem in that the produced polycarbonate contains foreign matter as an impurity. The reason for the presence of such foreign matter in the produced polycarbonate has not been elucidated yet, especially as to whether it is generated during the reaction or it is inadvertently introduced through the raw materials or through the materials of the production equipment. It has generally been attempted to remove the foreign matter from the obtained polycarbonate. However, since the viscosity of a molten polycarbonate is high, it is difficult to remove the foreign matter, especially foreign matter particles having an extremely small size, from the polycarbonate. When the obtained aromatic polycarbonate is used in the optical application field, especially in the production of an optical disk, the presence of extremely small foreign matter particles contained in the polycarbonate creates a serious optical defect, causing a bit-error in optically reading information recorded on a shaped article produced from the polycarbonate.
On the other hand, it is known that discoloration of a polycarbonate is greatly influenced by the material of a portion of the reaction equipment (such as a reactor, a pipe and the like) which is brought into contact with a molten polymer (such a portion is hereinafter, frequently referred to as a "molten polymer-contacting portion"). Also, it has been pointed out that, when a reactor made of stainless steel is used in producing a polycarbonate, a discoloration of the polycarbonate inevitably occurs. Therefore, it has been proposed to use a reactor produced from a material other than stainless steel. For example, U.S. Pat. No. 4,383,092 proposes a method in which tantalum, nickel or chromium is used as a material for a reactor in order to prevent the polycarbonate from suffering discoloration. Unexamined Japanese Patent Application Laid-Open Specification Nos. 4-72327 and 4-88017 propose reactors, respectively, made of a material having a chromium content or nickel content of 85% by weight or more, and a material having an iron content of 20% by weight or less. However, the materials disclosed in these patent documents have problems in that, due to their low iron content, these materials are not easily available and have poor workability or machinability.
To alleviate the above-mentioned problems, various attempts have been made. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 6-345860 discloses a method in which a polymerization is performed in a two-stage process comprising a prepolymerization and a main polymerization, wherein, as a material for a molten polymer-contacting portion of the reaction equipment, a material having an iron content of 20% by weight or less is used in the prepolymerization stage and a material having an iron content of more than 20% by weight is used in the main polymerization stage. In this method (in which the use of a material having an iron content of 20% by weight or less is limited to the prepolymerization stage), the use of a material having a low iron content can be reduced. However, in this method, it is requisite to use a material having an iron content of 20% by weight or less for the entire region of the molten polymer-contacting portion of the reaction equipment employed in the prepolymerization stage. This requirement still poses great problems since such material having a low iron content is not easily available and has poor workability or the like.
Unexamined Japanese Patent Application Laid-Open Specification Nos. 4-7328 and 4-7329 propose an improved method of using stainless steel as a material for a reactor. Specifically, in this method, the surface of the stainless steel, which contacts the molten polymer, is subjected to buffing and/or acid washing. However, this method using buffing and/or acid washing cannot satisfactorily prevent the discoloration of a produced polycarbonate.
In Unexamined Japanese Patent Application Laid-Open Specification No. 6-56984, the present inventors previously proposed a production method for a polycarbonate, using a stainless steel reactor, wherein a molten polymer-contacting portion has been treated with a cleaning liquid comprising an aromatic monohydroxy compound. This method has enabled a desired polycarbonate to be produced with a reduction in discoloration. However, this prior art document has no teaching or suggestion about how to produce a polycarbonate which is free of the above-mentioned foreign matter. As a method for producing an aromatic polycarbonate with a low foreign matter content by the melt transesterification process, Unexamined Japanese Patent Application Laid-Open Specification No. 5-239334 (corresponding to EP 615996 A1) discloses a method in which an aromatic dihydroxy compound and a carbonic diester are subjected to a melt transesterification in the presence of a catalyst to obtain a polycarbonate in the molten state, and additives are added to and kneaded with the obtained molten polycarbonate before solidification thereof, and, optionally, the resultant kneaded polycarbonate composition is subjected to filtration by using a polymer filter, to thereby obtain a polycarbonate having a low foreign matter content, which can be used for producing optical articles. (In this method, the addition of additives before solidification of the obtained molten polycarbonate is intended to reduce the chance of occurrence of foreign matter by reducing the number of times that the obtained polycarbonate is melted.) However, this method has a problem in that, although the content of foreign matter having a relatively large particle diameter, namely, 1 .mu.m or more, can be reduced, the content of foreign matter having a particle diameter as small as less than 1 .mu.m cannot be satisfactorily reduced. Moreover, it is economically and practically disadvantageous to use a high precision polymer filter having a filtration cut-off size of less than 1 .mu.m; the reasons for this reside in that, when a molten polycarbonate (inherently having a high melt viscosity) is forcibly passed through such a high precision filter by using an extruder, the load sustained on the extruder is extremely large, and also that such high precision filter is likely to be clogged. In fact, the polymer filter used in the working examples of the above-mentioned patent document has a filtration cut-off size larger than 5 .mu.m, and there is no description therein concerning the content of foreign matter having a size smaller than 1 .mu.m.
Unexamined Japanese Patent Application Laid-Open Specification No. 6-234845 (corresponding to U.S. Pat. No. 5,525,701) discloses a method in which an aromatic dihydroxy compound and a carbonic diester are subjected to a successive melt transesterification by using at least two reactors which are connected in series, wherein each of the final reactor and a reactor immediately preceding the final reactor has at least one polymer filter provided at an outlet thereof. However, in this method, the polymer filter equipped at the outlet of the final reactor has a filtration cut-off size of 5 .mu.m or more, so that the content of foreign matter having a size of less than 1 .mu.m cannot be reduced.
Unexamined Japanese Patent Application Laid-Open Specification No. 7-207015 teaches that, when an aromatic dihydroxy compound and a diaryl carbonate are subjected to melt transesterification in the presence of lithium phthalimide, side reactions can be suppressed, so that suppression can be achieved to some extent with respect to the formation of foreign matter which is generated by a branching reaction and is insoluble in a solvent, such as methylene chloride. By this method, the content of foreign matter having a size of 1 .mu.m or more can be reduced; however, by this method, the content of foreign matter having a size of less than 1 .mu.m cannot be reduced to a satisfactorily low level.
As is apparent from the above, a method is not known at all which can be used for producing a transesterified polycarbonate in which the content of extremely small foreign matter having a size of less than 1 .mu.m is reduced to a satisfactorily low level.